George Zimmerman, after his arrest by Seminole County Sheriff's Department in Sanford, FloridaSeminole County Sheriff's Department

The Justice Department has decided against filing civil federal rights charges against George Zimmerman for the 2012 shooting of black teenager Trayvon Martin.

Thursday 26 February marks three years since the 17-year-old was shot dead. Federal prosecutors have ruled that there is unsufficient evidence that Zimmerman violated Martin's civil rights, according to the report.

Martin was unarmed and was shot walking back to his Florida home holding Skittles and an iced tea.

Zimmerman, who was working at the time as a neighbourhood watch volunteer, was acquitted in 2013 of second-degree murder and manslaughter in the shooting of Martin.

Zimmerman, who is white, has said he acted in self-defence when he shot 17-year-old Martin during an alteracation inside a gated community in Florida, just outside Orlando.

The case raised questions of racial bias over the killing, with demonstrations across the US. More than 600,000 people signed an online petition asking the Department of Justice to press civil rights charges against Zimmerman.

The Washington Post had reported last October that three officials - speaking on condition of anonymity - said it was almost certain that the department would close the investigation.

One of the officials, said that investigators only wanted to "dot their i's and cross their t's".

That investigation's prime issue was whether the killing amounted to a federal civil rights violation, which would have required proof that it was motivated by racial animosity. Martin's parents have said Zimmerman started the fight and racially profiled Martin. However, the Justice Department said there was not enough evidence to bring federal civil rights charges, which would have needed proof that the killing was motivated by racial animosity.

"This decision is limited strictly to the department's inability to meet the high legal standard required to prosecute the case under the federal civil rights statutes; it does not reflect an assessment of any other aspect of the shooting," the Justice Department said.